1. Field of the Invention
The present invention relates generally to a magnetic information storage medium and more particularly to such a medium having a reference feature marked on its surface for orienting a magnetic read/write head and for allowing track zero to be defined.
2. Description of the Prior Art
The track density of magnetic storage disks for conventional floppy disk drives is approximately forty-eight to one hundred thirty-five tracks per inch (TPI). In contrast, optical disk drives are capable of achieving track densities in excess of 15,000 TPI. These higher track densities are achieved through the use of closed loop optical servos that allow an optical read/write head to follow data track eccentricities caused by defects in the medium and by disturbances from outside forces. In rigid type magnetic disk drives, track densities of up to 1500 TPI are presently used. These drives commonly have multiple disks in which both sides are used for data. To achieve the high track density a dedicated surface of one of the disks is used for magnetic track servo information.
In order to utilize the multiple data tracks on flexible and rigid magnetic disk drives, a method for labeling the various data tracks is required. Typically, data storage disk drives have a location called track zero from which all other tracks are referenced. In conventional floppy disk drives, the track zero position is set by an adjustment of the stepper motor when the motor is locked at phase zero. Each data track is then a subsequent number of steps from the zero phase. Track zero is typically on the outer radius of the diskette. Since the stepper motor that positions the carriage is run in an open loop mode (no position feedback from the disk), the track zero position must be set very accurately from drive to drive. If not, the track zero location written on a disk by one drive might be in a different position than the track zero location on a disk written by a different drive. This could eliminate the interchangeability of media between disk drives.
Rigid media type disk drives (e.g. Winchester type drives) have track densities approximately ten times that of conventional floppy drives. A small error in detecting the position of track zero could result in a large offset of the read/write head to the data track. This is usually not a problem, however, since the media is formatted on the same drive that it is used in. Media is never removed from the drive so track zero position errors do not propagate.
Drives having multiple recording heads driven by a single positioning mechanism normally rely on a single reference device for determining track zero. Temperature, humidity and mechanical effects will cause both the media and head mechanisms to change their initial relationships with time and thus limit the maximum track density that can be achieved. If the media is removable the problem is compounded by the mechanical alignment tolerance from drive to drive.
In U.S. patent application Ser. No. 07/202,719, filed June 3, 1988, an apparatus and method are disclosed for inscribing physical features on a surface of a magnetic medium for providing optical servo tracking information.
At least two techniques have been reported for utilizing a mark inscribed in the metal oxide coating of a magnetic disk. Guglielmino, in U.S. Pat. No. 4,584,641, issued on Apr. 22, 1986, discloses a technique for preventing unauthorized copying of a program recorded on the magnetic disk in which a permanent mark is made on the magnetic disk, such as by scratching the metal oxide layer. Similarly, Brotby, in U.K. Patent Application No. 2,131,580 A, filed Nov. 1, 1983, discloses a technique for placing a permanent defective area on the surface of a magnetic disk for preventing unauthorized copying of a program recorded on the disk.
Various techniques have been reported for using optical means for acquiring track following servo information contained on a magnetic recording medium.
For example, Ahn, et al., in U.S. Pat. No. 4,633,451, issued on Dec. 30, 1986, for "Optical Servo For Magnetic Disks", discloses the use of a laser diode to read track following servo information in the form of a plurality of spots contained in an optical layer positioned above a magnetic recording layer.
DiStefano, et al., in U.S. Pat. No. 4,570,191, issued on Feb. 11, 1986, for "Optical Sensor For Servo Position Control", discloses a servo sensor comprising a light source and a light detector, axially aligned and contained on a single semiconductor chip.
M. Johnson, in U.S. Pat. No. 4,558,383, issued on Dec. 10, 1985, for "Information Storage Disk Transducer Position Control System Using a Prerecorded Servo Pattern Requiring No Alignment With The Storage Disk", discloses a servo apparatus having a sensor for detecting a pattern of spots on a surface of an information storage medium. The spots comprise a dense array of substantially translation invariant marks and separate information recording tracks are detected by measuring the rate at which the spots are detected.
J. Cocke, et al., in U.S. Pat. No. 4,587,579, issued on May 6, 1986, for "System for Position Detection On A Rotating Disk", disclose a servo control system comprising a detector for reading a plurality of spiral radial-position-encoding patterns on a medium.
A. S. Hoagland, in "Optical Servo Of Magnetic Recording", IBM Technical Disclosure Bulletin, Vol. 20(10), page 4108 (March 1978), suggests a system for achieving optical servo control where a flexible disk medium includes a plurality of optical servo tracks positioned underneath a magnetic layer.
D. A. Thompson, et al., in "Embossed Servo Techniques For Floppy Disks", IERE Conference Proceedings, No. 43, page 321 (July 1979), suggest the use of embossed marks on flexible magnetic media for obtaining non-magnetic optical or capacitive servo information.
N. Koshino and S. Ogawa, in "Optical Method Of The Head Positioning In Magnetic Disk Systems", preprint from IEEE Transactions on Magnetics (1980), discloses an optical head for achieving track following servo control which is mounted on the head arm and which includes an LED light source and three optical fibers for delivering light to a medium. The medium comprises a plurality of circular optical tracks, dyed black, and located underneath a magnetic film.
In U.S. patent application Ser. No. 07/178,542, filed Apr. 7, 1988, an optical servo tracking head is disclosed for reading optical servo tracking information contained on magnetic media comprising nonreflecting servo areas situated around reflective land areas.
Related work has occurred in the laser video disk area, from which optical disks for digital data storage and the audio laser disk (CD) have evolved. A laser and associated optics are used to acquire servo information as well as read data from the disk. The data can be inscribed during a mastering process as in video and audio disks or it can be written by the read/write laser as in disks for digital information storage.
K. D. Broadbent, in "A Review of the MCS Disco-Vision System", Journal of the SMPTE (1974), describes the Laser Video mastering techniques as well as the servo and read back methods. The mastering machine uses an argon laser to ablate pits in a metallic layer which is deposited on a glass plate. Disks are replicated from the master which contain servo information as well as the video data. A technique for deriving the servo information is described.
None of these techniques describe a method for utilizing the presence of a reference feature on the surface of a magnetic medium for directing a magnetic read/write head or an optical servo head to a position from which a plurality of data tracks can be referenced.